Pulmonary valved conduits, used to replace diseased arteries such as the pulmonary artery, have been used for the last thirty years. These pulmonary valved conduits have been generally effective at treating pulmonary atresia, pulmonary stenosis, or pulmonary insufficiency. In early embodiments, the conduits consisted of crimped woven or knit polyester tubes housing a valve fabricated from metal components or metal/plastic polymer components. Later valves consisted of metal frames suspending ceramic flat or curved (convex or concave) discs or hemidiscs that were passive to the flow of blood exiting the chambers of the heart.
There were at least two disadvantages of these pulmonary valved conduits. First, there was a need to preclot the polyester tubes in order to preclude blood from seeping through the pores of the knit or woven material. Second, after months of usage, a layer of fibrinous material formed in the inner lumen over time from the accumulation of blood proteins. This layer of fibrinous material, referred to as "intimal peel," often separated from the inner wall and interfered with the effective operation of the valve. An ineffective valve required re-operation to replace the conduit and valve, if caught in time. In some instances, if valve replacement came too late, the results were fatal.
In the case of a mechanical valve made of metals and ceramics, there was also the need to continuously medicate the patient with anticoagulants for the duration of his life. That is because no man-made material that is impervious to the clotting effects of blood exists today. Anticoagulation of the patient prevents coagulation of blood on the surfaces of the inner lumen and on the sinuses of the cusps of the valves. Moreover, for children suffering from congenital pulmonary atresia or pulmonary insufficiency, the use of pulmonary valved conduits were not always effective. As the child grew, the prosthetic was less effective due to its inadequate size, requiring frequent replacement. Also, for children especially, anticoagulation requirements are difficult to regiment.
Beginning in the early 1970s, biological valves were used within the crimped polyester conduits to produce a hybrid prosthetic referred to as a biological valved conduit. It was believed that a biological valve, such as a porcine aortic valve fixed with formaldehyde or glutaraldehyde, would eliminate the need for lifetime anticoagulation administration. While the desired result was achieved, there was still the problem of separation of the "intimal peel" from the inner lumen. The intimal peel clogged the leaflets of the biological valve. Moreover, while a porcine aortic valve functioned relatively well as a replacement aortic valve, where the blood pressures from the left ventricle are substantially high (80-120 mmHg), it did not function nearly as effective as a replacement pulmonary valve, where the blood pressures from the right ventricle are quite low (0-15 mmHg). The porcine aortic valve leaflets, which are relatively thick, become substantially less flexible when "fixed" by chemical solutions. Opening at low pressures becomes more difficult. Also, the flow therethrough becomes turbulent, which undesirably promotes the degeneration of the biological material.
The disadvantages mentioned above can be minimized or overcome by using a naturally formed biological conduit which integrates a valve suitable for pulmonary replacement. Preferably, the valve opens and remains open with minimal resistance to the flow of blood at relatively low pressures of less than 1 mm Hg. The valve should also preferably close under the effect of minimal backflow of blood and be capable of withstanding backflow pressures of up to 200-300 mm Hg. An example of a naturally-formed valvular conduit is described in U.S. Pat. No. 5,500,014 to Quijano, et al., the entire specification of which is incorporated herein by reference.
Even this arrangement is limited in that the naturally formed donor conduits have normally minimum diameters of about 22 mm. The largest naturally occurring biological valved conduits are veins with venous valves found in the jugular veins of caprine, cervine, canine, ovine, bovine, equine and other quadruped species and marsupials (e.g., kangaroos and wallabies). While children may be treated effectively with pulmonary valved conduits having such diameters of 22 mm or less, many adolescents and adults require larger-diameter prosthetics. The flow capacity of a pulmonary conduit exiting the right ventricle must be large enough to permit oxygenation of the blood at a sufficient rate to maintain systemic perfusion with blood returning to the heart from the lungs. With normal pulmonary circulation, the blood exiting the right heart through the pulmonary artery (trunk) divides into two separate flows: the right pulmonary artery servicing the right lung, and the left pulmonary artery servicing the left lung. That requires that each lung receive enough flow of blood through each of the two pulmonary arteries emanating from the pulmonary trunk. That flow capacity often requires a diameter greater than the 22 mm found in the jugular veins of donor animals.